Police have arrested three doctors at a private clinic in Milan who allegedly performed unnecessary, and sometimes fatal, surgery in an attempt to boost their fees.

Eleven other members of staff at the Santa Rita clinic are also accused of being part of a ring suspected of killing five patients as a result of operations that were not needed. Officers said the medical team carried out 91 unnecessary operations.

One of the patients, an 85-year-old man with suspected lung cancer, died during exploratory surgery when a simple biopsy would have sufficed.

Investigators said payouts from the Italian national health service for useless operations totalled €2.5m over two years, while staff were able to increase their wages from €1,700 to €27,000 a month.

A suspect was overheard saying on a police wiretap: "It's obvious that the moment you are told 'the more you operate, the more you are paid', you are going to be more aggressive in surgery."

The owner of the clinic and nursing home, Francesco Pipitone, is among those under house arrest, while two of the doctors suspected of killing patients, Pierpaolo Brega Massone and Pietro Fabio Presicci, are in jail. Interrogations of the suspects began yesterday.

Brega Massone, who transferred from the clinic before he was arrested, is also suspected of killing an elderly woman by forcing her to undergo three profitable operations to remove a tumour when one would have sufficed.

Ten patients with tuberculosis had lungs removed, with one staff member telling a colleague "cases were treated as tumours, even if the patients had TB". One patient, when contacted by police, said she had no idea part of her lung had been removed. Fifteen women had breasts removed because of suspected tumours, when biopsies would have been sufficient, investigators said.

Officers began using wiretaps after an anonymous tip-off last year. One doctor was overheard describing how he took scant notice of hygiene procedures, claiming, "if the patient is 90 or 95 he has very little time left to live".

Prosecutor Micaela Curami said she was "stunned by the absence of any consideration for the patients or for their suffering, which was not relieved but actually increased".

One patient, Giovanni Rizzitano, 64, told the Italian daily newspaper Corriere della Sera how he escaped from the hospital. After going in to have a minor bladder problem treated he woke up on the operating table to find half his lung had been removed. "I asked them what on earth they had done, and they said they wanted to take out my thyroid next. So I fled," Rizzitano said.